As is well known in the art, an internal combustion engine, such as a gasoline engine, is used to drive a car, and the power of the reciprocating operation of a 4 or 6 cylinder engine is transmitted to the wheels from one end of the engine crank shaft, and auxiliary machinery, such as an alternator, usually is driven by the other end of the crank shaft through a pulley and one or more belts.
Vibromotive force is applied to the crank shaft, due to inertia forces of the engine and the combustion force of the cylinders, and the shaft's rotary motion is subjected to torsional vibration, as is long known in the art. Therefore, a so-called "torsional damper" has been developed to restrict the vibration of a car body and the noise to the passenger compartment resulting from such torsional vibration.
It has been technically difficult to arrange a torsional damper in the power transmission path at the wheel-driving end of the crank shaft because of obvious complications. Therefore, since a pulley hub and a pulley mechanism for the transmission of power to auxiliary machinery, such as an alternator, is disposed at the other end of the shaft, a torsional damper has been arranged around the pulley hub by utilizing its relatively open mechanism.
As shown in FIG. 14 of the accompanying drawings, in such a prior arrangement a pulley hub 3 usually is fastened by a screw 2 and a key (not shown) to the tip of a crank shaft 1 in such a manner as to be capable of transmitting torque and to prevent the hub from falling off the shaft. A damper mass 5, which has a belt groove 4 for the transmission of power, via a belt (not shown) to auxiliary machinery, such as an alternator, is disposed about the pulley hub 3, and damper rubber 6 is interposed between the damper mass 5 and the pulley hub 3 and fixed to both, as by baking, thereby forming a torsional damper 7.
It is well known in the art, however, that in a 4 or 6 cylinder engine or the like, bending vibration also is generated in the crank shaft due to inertia forces and the combustion forces because the pins and journals of the crank shaft 1 are not arranged symmetrically in an axial direction. In conventional engines, however, not only the cylinder block but also the crank shaft is extremely heavy and has extremely large rigidity. Therefore, even when bending vibration is generated, it is not very great. In recent years, however, the weight of the crank shaft has been reduced in order to improve fuel consumption and to control torque more accurately, while horse power has been increased. These two factors together increase bending vibration. Thus, a dynamic damper for bending vibration has been earnestly desired from the technical aspect of both restricting the vibration of and suppressing noise in the car body.
When the frequency of bending vibration is plotted on the abscissa and the amplitude on the ordinates, as shown in FIG. 15, a peak of vibration appears at a predetermined frequency f.sub.o, such as represented by the curve C.sub.1. Moreover, it has been shown that uninodal vibration Q.sub.1 and binodal vibration Q.sub.2 also develop, as shown in FIGS. 16 and 17, respectively. A low-order bending vibration, such as the uni- and binodal vibrations, vibrates the cylinder block and is a significant factor for the occurrence of noise in the passenger compartment, to say nothing of the vibration of the car body. Further, when bending vibration becomes great, a crucial problem occurs in the strength of the crank shaft itself.
The conventional crank damper pulley structure, such as shown in FIG. 14, is primarily directed to obtain a torsional damper function to cope with the torsional vibration, and cannot be easily adapted to perform a dynamic damper function to cope with the low-order bending vibration described above.
The technique of Japanese Utility Model Laid-Open Publication No. 14139/1980, for example, disposes damper rubber inside a pulley hub but does not so dispose a damper mass. Therefore, this prior art device does not have a damping function against low-order bending vibration and, hence, its damping function cannot be fully effective.
In the damper pulley structure disclosed in Japanese Utility Model Laid-Open Publication No. 70554/1983, the torsional damper function is not separated from the dynamic damper function to cope with the bending vibration, and, hence, the performance of the dynamic damper cannot be fully effective.
The prior art device of Japanese Patent Laid-Open Publication No. 40060/1984 puts emphasis on the torsional damper function so that the dynamic damper function against bending vibration is not fully effective.